1. Field of the Invention
This invention relates to solar cells and particularly to an improved solar cell having essentially a convoluted, single P-N junction providing a plurality of generally parallel P-N junction planes separated by distances of the order of not over one diffusion length whereby carriers generated within the cell are more efficiently collected by the P-N junction of the cell. The P-N junction may be parallel to or perpendicular to the top surface of the cell or a combination of the two.
2. Description of the Prior Art
One of the factors limiting the efficiency of a solar cell is the fraction of photoelectrically produced minority carriers which reach the P-N junction of the solar cell before recombining. Surface and bulk recombination can both play important roles in the recombination of minority carriers generated within the solar cell. Even in the best of present day solar cells, approximately one-quarter to one-third of the generated carriers are lost through recombination.
In an N/P type solar cell, holes produced in the N-region may be lost by recombination in the bulk or at the front surface of the cell. Similarly, electrons produced in the P-region may recombine internally or at the rear surface of the solar cell. In order to minimize recombination in the N-region or at the front surface on which the solar radiation falls, the depth of the P-N junction (actually the N-region thickness) is generally made quite small, of the order of one-half micron. In this instance most photons will pass through this N-layer and be absorbed in the P-region thereby producing hole-electron pairs, with a major portion being generally absorbed in the forward portion of the P-region near the P-N junction. The major contributing cause of low collection efficiency then becomes bulk recombination of the electrons in the P-region, and surface recombination of holes at the electrical contact affixed to the rear of the solar cell. Generally speaking, all carriers generated in the back half of the P-region of the N/P solar cell will be lost by recombination since they are more likely to reach the back surface of the solar cell rather than the P-N junction. Some of these carriers do, however, reach the P-N junction but a corresponding fraction of those produced in the front half of the P-region also reach the back surface of the solar cell, with no net gain in efficiency. Of those carriers generated in the forward portion of the P-region which migrate towards the P-N junction, many are lost by bulk recombination. Fortunately for solar cell efficiency, the exponential absorption of light results in most of the carrier generation occurring near the front of the solar cell.
Carrier collection efficiency is, therefore, a significant problem in solar cell design. To date, approaches to solution of the problem have included making the lifetime, and therefore the diffusion length, for minority carriers in the P-region as high as possible, and/or introducing a graded distribution of impurities into the P-region near the P-N junction, thereby providing a built-in electric field which urges electrons towards the P-N junction. Even with these improvements approximately one-third of the generated carriers in a solar cell are lost through recombination.
A. I. Bennett in his U.S. Pat. No. 3,682,708, proposes a multilayer solar cell having one convoluted P-N junction to improve the efficiency of the cell. The construction of the cell requires numerous epitaxial growth, diffusion and masking processes. Repeated temperature excursions required for the process involved can detrimentally affect the initial substrate material, the epitaxial material subsequentially grown and the electrical characteristics of the numerous P-N junctions which are joined together to make the one convoluted P-N junction. Additionally, the processes involved inherently have a detrimental effect on the P-N junctions formed in that impurities can be trapped at the interfaces of alternate regions of opposite type conductivity and degrade the electrical characteristics of the P-N junction formed thereat.
An object of the invention is to provide a new and improved solar cell which overcomes the deficiencies of the prior art solar cells.
An object of this invention is to provide a solar cell having a P-N junction construction for improving the efficiency of collecting the photoelectrically produced minority carriers.
An object of this invention is to provide a solar cell having an improved resistance to nuclear particle radiation damage.
Another object of this invention is to provide a semiconductor solar cell having a convoluted P-N junction so as to present a plurality of generally parallel P-N junctions contained in the semiconductor body.
A further object of this invention is to provide a solar cell wherein one region is recrystallized semiconductor material having solid solubility of metal therein to impart a selected type conductivity and resistivity thereto.
A still further object of this invention is to provide a solar cell wherein a greater proportion of the P-N junction is perpendicular to the top surface of the solar cell.
Other objects of this invention will, in part, be obvious and will, in part, appear hereinafter.